All references cited in this specification, and their references, are incorporated by reference herein where appropriate for teachings of additional or alternative details, features, and/or technical background.
Disclosed in the embodiments described herein is a method and system for controlling a printing device's tone reproduction curve. Control of the tone reproduction curve may minimize contouring, help maximize the number of shades or colors available for an output image, and maintain the desired output color. While certain elements of this disclosure will be described in reference to a xerographic print engine, it is also amenable to other electrophotographic processes such as, for example, ionographic print engines and like applications.
Electrophotographic copiers, printers and digital imaging systems typically record an electrostatic latent image on an imaging member. The latent image corresponds to the informational areas contained within a document being reproduced. In xerographic systems, a uniform charge is placed on a photoconductive member and portions of the photoconductive member are discharged by a scanning laser or other light source to create the latent image. In ionographic print engines the latent image is written to an insulating member by a beam of charge carriers, such as, for example, electrons. However it is created, the latent image is then developed by bringing a developer, including colorants, such as, for example, toner particles into contact with the latent image. The toner particles carry a charge and are attracted away from a toner supply and toward the latent image by an electrostatic field related to the latent image, thereby forming a toner image on the imaging member. The toner image is subsequently transferred to a physical media, such as a copy sheet. The copy sheet, having the toner image thereon, is then advanced to a fusing station for permanently affixing the toner image to the copy sheet.
The approach utilized for multi-color electrophotographic printing is substantially identical to the process described above. However, rather than forming a single latent image on the photoconductive surface in order to reproduce an original document, as in the case of black and white printing, multiple latent images corresponding to each color separation are recorded on one or more photoconductive surfaces. The electrostatic latent image for each color separation is developed with toner of that color. Thereafter, each color separation is ultimately transferred to the copy sheet in superimposed registration with the other toner images, creating, for example, a multi-layered toner image on the copy sheet. This multi-layer toner image is permanently affixed to the copy sheet in substantially conventional manner to form a finished copy.
FIG. 1 is a simplified elevational view of essential elements of one type of a color printer, showing a context in which embodiments of the present disclosure may be utilized. Specifically, there is shown an “image-on-image” xerographic color printer, in which successive primary-color images are accumulated on a photoreceptor belt, and the accumulated superimposed images are in one step directly transferred to an output sheet as a full-color image.
Specifically, the color printer of FIG. 1 includes a belt photoreceptor 10, along which are disposed a series of stations, as is generally familiar in the art of xerography, one set for each primary color to be printed. For instance, to place a cyan color separation image on photoreceptor 10, there is used a charge corotron 12C, an imaging laser 14C, and a development unit 16C. For successive color separations, there is provided equivalent elements 12M, 14M, 16M (for magenta), 12Y, 14Y, 16Y (for yellow), and 12K, 14K, 16K (for black). The successive color separations are built up in a superimposed manner on the surface of photoreceptor 10, and then the combined full-color image is transferred at transfer station 20 to an output sheet. The output sheet is then run through a fuser 30, as is familiar in xerography.
Also shown in FIG. 1 is a set of what can be generally called “monitors,” such as 50 and 52, which can feed back to a control device 54. The monitors such as 50 and 52 are devices which can make measurements to images created on the photoreceptor 10 (such as monitor 50) or to images which were transferred to an output sheet (such as monitor 52). These monitors can be in the form of optical densitometers, calorimeters, electrostatic voltmeters, etc. There may be provided any number of monitors, and they may be placed anywhere in the printer as needed, not only in the locations illustrated. The information gathered therefrom is used by control device 54 in various ways to control in the operation of the printer, whether in a real-time feedback loop, an offline calibration process, a registration system, etc.
An image to be rendered (an input image) is received in the form of, or is transformed into the form of, a set of contone values. For example, each contone can have a value ranging from 0 to 255 (in eight bit systems) or from 0 to 4095 (in higher resolution twelve bit systems). The contone values are indicative of how much colorant should be applied to the output medium in order to render a small portion of the image. For example, 255 may indicate that no colorant should be applied to a small portion of the medium and a contone value of zero may indicate that the entire area associated with a halftone cell should be covered with toner. An Engine Response Curve (ERC) gives the relationship between the amount of mass developed to the paper or to an internal media and the contone gray level. If the ERC is not optimal, a tone reproduction curve (TRC) can be incorporated.
The tone reproduction curve (TRC) modifies the input gray level before it is sent to the print engine. The TRC is adjusted in order to maintain a stable system reproduction curve (SRC). The resulting image processing system TRC changes the input gray level to obtain a stable SRC. In particular, image processing system tone reproduction curves may be created during the product development phase and stored in data files on the actual device, or, for example, in the accompanying driver or software files. Therefore, each possible mode and each possible combination of image adjustment, such as contrast and brightness, has an associated image processing system tone reproduction curve stored in a data file. The data file corresponding to the image input terminal information and the image output terminal information is then referenced and applied by the image processing sub-system to the input image information.
Some electrophotographic systems include a hierarchical control scheme in an attempt to provide an actual system reproduction curve that is as close as possible to the ideal or target system reproduction curve. For example, some electrophotographic systems include what are referred to as level 1 control loops for maintaining electrophotographic actuators at associated set points, level 2 control loops for selecting set points for the level 1 control loops. These give a stable ERC. Level 3 controls adjust the TRC if ERC stability cannot be maintained to achieve a stable SRC.
Xerographic actuators include, for example, cleaning field strength or voltage, development field strength or voltage, imager or laser power, and AC wire voltage associated with some developers. In some xerographic environments level 1 control loops include electrostatic voltmeters for measuring charge voltage generated by charge applied to a photoconductive member. For instance, the electrostatic voltmeters measure the charge applied in the area of test patches in inter-document or inter-page zones of the photoconductor. If measured voltages, such as, for example, a discharged area voltage, or a cleaning voltage of an area surrounding a discharged area deviate from set point values, level 1 control loops adjust xerographic actuators to return the measured voltages to set point potentials. For example, the level 1 control loops vary a charge or bias voltage applied to elements of a developer to adjust a resulting development field and/or cleaning filed. Additionally, the level 1 control loops may adjust a laser power to return a related discharge field back toward a discharge field set point.
The sensors for level 2 control loops include, for example, infrared densitometers. In xerographic environments, and perhaps in other electrophotographic environments, infrared densitometers are also known as Enhanced Toner Area Coverage Sensors (“ETACS”). The infrared densitometers or ETACS are used to measure, for example, the density of toner or colorant applied to or developed on the photoconductive member.
ETAC systems generally are designed to measure both specular and diffuse reflected light, calibrating the specular read using the diffuse read. The diffuse reflectance increases proportionally to the area coverage of toner on the surface of the photoreceptor and continues to increase as the toner coverage grows past a monolayer. The specular reflectance decreases proportional to the area coverage, and saturates at a lower response as the toner layer grows past a monolayer.
Typically, a printer using control systems which rely on monitors such as 50, 52 of FIG. 1 require the deliberate creation of what shall be here generally called interdocument zone patches which are developed and subsequently measured in various ways by one or another monitor. These test marks may be in the form of test patches of a desired darkness value, a desired color blend, or a particular structure, such as a line pattern; or they may be of a structure particularly useful for determining registration of superimposed images (“fiducial” or “registration” marks). Various image-quality systems, at various times, require test marks of specific types to be placed, with respect to FIG. 1, on photoreceptor 10 at specific locations. These interdocument zone patches may be made on photoreceptor 10 by one or more lasers such as 14C, 14M, 14Y, and 14K. As is familiar in the art of “laser printing,” by coordinating the modulation of the various lasers with the motion of photoreceptor 10 and other hardware (such as rotating mirrors, etc., not shown), the lasers discharge areas on photoreceptor 10 to create the desired test marks, particularly after these areas are developed by their respective development units 16C, 16M, 16Y, 16K. The test marks must be placed on the photoreceptor 10 in locations where they can be subsequently measured by a (typically fixed) monitor elsewhere in the printer, for whatever purpose.
Methods for making such interdocument zone patches are known. For example, U.S. Pat. No. 6,526,240 is directed toward a versatile system for causing the printing hardware to create test marks of desired types, in desired locations on the photoreceptor or on an output sheet, on demand.
A set of test patches may be written in an interdocument or interpage zone on the photoconductor. The test patches are then developed and the amount or density of colorant or toner present in the test patches is measured. If the amount of colorant or toner in a test patch is incorrect or varies from a target test patch density, the level 2 control loops generate or select one or more new set points for the xerographic actuators of the level 1 control loops. For instance, if a high-density test patch, such as a test patch corresponding to a target density of 100 percent (e.g., contone value zero), includes too little colorant or toner (is less dense than the target density), then the level 2 control loop may increase a set point related to the generation of a development field.
If the measured or actual density of a low-density test patch, or a test patch associated with a low-target density, such as, for example, 10 percent (e.g., a contone value of 25 or 26), includes more colorant or toner than is indicated by the associated target density, the level 2 controls may select or determine a new set point for a level 1 control loop associated with controlling a cleaning field voltage. For instance, increasing the cleaning field may reduce a toner density measured in a next low-density test patch. If an infrared densitometer measures a deviation from a midrange target density in an associated test patch, the level 2 controls may select or determine a new set point for a level 1 controller responsible for regulating laser power.
The level 2 control loops strive to maintain the actual densities of test patches at desired or target levels. The assumption is that by adjusting the level 1 actuator set points to maintain the densities of a few test patches at target levels, an entire actual TRC will be maintained at or near an ideal or target TRC.
FIG. 2 is a plan view of a portion of photoreceptor 10. Within a printer such as shown in FIG. 1, the photoreceptor 10 will move in a process direction P. At any arbitrarily chosen location on the photoreceptor 10, there can be considered what is called an “imageable area” indicated as A. This imageable area may, but need not, correspond in some way to an area on which an image desired to be printed is placed (including a predetermined interdocument zone); it may, but need not, correspond to one or another physical “landmark” formed in or on photoreceptor 10, such as a seam or hole; indeed, the entire surface of the photoreceptor 10 may be considered the imageable area. However, the imageable area must define relative thereto an “origin” point, such as shown as (0, 0) in FIG. 2, from which any other point within the imageable area can be located, such as shown as (x, y). The coordinate system thus enabled can facilitate locating a desired test mark essentially anywhere in the imageable area. Numerous types of test marks may be used. In general, such marks frequently comprise different configured patches as set forth, for example, in FIGS. 3-5 of U.S. Pat. No. 6,526,240, commonly assigned.
Errors or deviations from the target TRC of the actual reproduction curve lead to errors in gray scale or color of images in output documents.
Some electrophotographic systems include a third level of control. Level 3 control loops may share the infrared densitometers of the level 2 control loops. Alternatively, level 3 control loops can include other sensors.
To implement level 3 control, a plurality of additional test patches are developed in inter page zones of an imaging member. The plurality of level 3 test patches is associated with a plurality of target level 3 test patch densities. The plurality of target level 3 test patch densities may or may not include the high, low and midrange target test patch densities described above. The level 3 controls use this information to build color correction lookup tables to be used in an image path of the system.
Many kinds of electrophotographic machines use multiple sensors, such as ETAC sensors, to monitor the developed mass. These sensors may monitor the developed mass for different colors (including just black) and different area coverages to infer the shape of the TRC and maintain color uniformity. ETAC systems may be calibrated by printing a series of test patches of different masses, measuring the masses directly by sucking off and weighing the toner and also monitoring the ETAC response. From these measurements, one can plot the measured mass versus the ETAC response, the plot being used as a calibration curve to infer the mass for any ETAC response. ETAC systems measuring both specular and diffuse light add to the cost of electrophotographic machines.
It would be useful to use a sensor with a more general sensing capability than an ETAC, yet a sensor that retains the ability to monitor the developed mass at low and high area coverages.